Mowing the lake: Tackling invasive lake weeds in Lake Rotoehu
On a still morning in Kennedy Bay at Lake Rotoehu, the Toi Moana Bay of Plenty Regional Council weed harvester moves across the water, slicing through dense mats of invasive aquatic weeds.
This purpose-built machine, often described as a ‘lawnmower of the lake,’ cuts down the invasive weed hornwort by one metre, before using a conveyor belt to lift the weed ‘clippings’ onboard. Each load collects up to 3.5 tonnes before the harvester returns to shore and transfers its haul to a waiting truck.
“We started harvesting lakeweed in 2006 and in 2012 we purchased our own harvester and have been 'mowing’ the lake ever since,” says Regional Council Lakes Operations Officer Justine Randell.
“Due to exceptionally high lake levels, the last year that Lake Rotoehu was able to be harvested was in 2022. This year’s harvest, which ran for three weeks from the end of March, resulted in 330 tonnes of dewatered weed being processed.”
Because it is relatively shallow, Lake Rotoehu has long struggled with the impacts of invasive aquatic weed species. Hornwort in particular covers a large area near the outlet to the warm geothermal waters of the Waitangi Soda Springs where the harvester was most recently operating.
Hornwort is a fast-growing aquatic weed that can grow up to 10m deep. Due to its invasive nature, it inhibits growth of native flora and fauna and degrades water quality by absorbing nutrients such as phosphorus and nitrogen (which can cause toxic algal blooms). Further to this, free-floating mats of hornwort have previously been known to clog up intakes to hydro-electric dams, creating a risk to the operation of this critical infrastructure.
By removing hornwort, it allows our native species to grow, helps improve water quality and makes the lake a nicer place for recreational purposes.
Circular thinking: Turning lake weed into energy and fertiliser
Removing the weed from Lake Rotoehu is only half the story. In partnership with Ecogas, the harvested hornwort is then transported to the Ecogas site in Reporoa, where it’s transformed into biogas and a bio-organic fertiliser through an anaerobic digestion process.
Ecogas North Island Manager Andy Bedford explains that the anaerobic digestion process is where microorganisms – much like those in a human stomach – break down the weed over a period of 60 to 70 days.
“Once the lake weed has gone through digestion, we’re left with a liquid fertiliser rich in nutrients, including nitrogen and potassium. That digestate is spread on local dairy farms, giving farmers an organic alternative to synthetic fertilisers.”
The biogas generated during this process also powers the Ecogas facility and is fed into the national gas grid by First Renewables, so it’s a win for local farmers, the environment and the broader energy system.
Understanding the bigger picture
While these efforts are a critical part of managing lake health, they are not a silver bullet. Invasive weed removal alone won’t fully resolve the complex issue of water quality in Lake Rotoehu.
University of Waikato Te Aka Mātuatua | School of Science PhD Senior Research Officer Whitney Woelmer has been studying the various drivers of water quality in the Rotorua Te Arawa lakes. She highlights the need for a broader understanding of what drives freshwater algal blooms – some of which can be toxic for fish, animals and humans.
“We know factors such as air temperature and nutrient levels play a huge role in the development of algal blooms. When the air temperature goes up, and phosphorus levels in the lake rise, it can create the perfect conditions for blooms.
“Until now, we haven’t had a lot of data on the impact of lake weed on nutrient dynamics in the lake, so being able to quantify how much weed is coming out of the lake will give us a better picture of how much the nutrients in the weed might impact algal blooms.”
The future of the lake
Ms Randell says the weed harvesting programme is an important part of restoring the health of the lake, but it’s only one piece of a much larger puzzle.
“While invasive weed removal is essential for reducing nutrient loads and improving water quality, it must be combined with ongoing research and broader catchment efforts to create a more sustainable future for the lake.
“Ultimately tackling the immediate lake quality issues, while investing in innovative, long-term solutions, is the strategy we are all working towards to help protect the lake for generations to come.”
